The present disclosure relates to a fuel cell stack assembly in which the mechanism for securing the fuel cell stack in its compressed, assembled state includes a spring bar loading a disc spring at its inner diameter and a compression band which circumscribes the fuel cell stack assembly.
Fuel cells convert fuel and oxidant to electricity and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (‘MEA’) consisting of a polymer electrolyte membrane (‘PEM’) (or ion exchange membrane) disposed between two electrodes comprising porous, electrically conductive sheet material and an electrocatalyst disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In typical fuel cells, the MEA is disposed between two electrically conductive separator or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates also act as current collectors and provide mechanical support for the electrodes.
Two or more fuel cells can be connected together in series to form a fuel cell stack to increase the overall voltage of the assembly. In a fuel cell stack, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell.
The fuel cell stack typically further includes manifolds and inlet ports for directing the fuel and the oxidant to the anode and cathode flow field passages respectively. The fuel cell stack also usually includes a manifold and inlet port for directing a coolant fluid, typically water, to interior passages within the fuel cell stack to absorb heat generated by the exothermic reaction in the fuel cells. The fuel cell stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant stream exiting the fuel cell stack.
In conventional fuel cell stack assembly designs, such as, for example, those described and illustrated in U.S. Pat. Nos. 3,134,697; 3,297,490; 4,057,479; 4,214,969; and 4,478,917, the plates which make up each conventional fuel cell assembly are compressed and maintained in their assembled states by tie rods. The tie rods extend through holes formed in the peripheral edge portion of the stack end plates and have associated nuts or other fastening means assembling the tie rods to the stack assembly and compressing the end plates of the fuel cell stack assembly toward each other. Typically the tie rods are external, that is, they do not extend through the fuel cell separator or flow field plates. One reason for employing a peripheral edge location for the tie rods in conventional designs is to avoid the introduction of openings in the central, electrochemically active portion of the fuel cells.
The peripheral edge location of the tie rods in conventional fuel cell designs however, has inherent disadvantages. It requires that the thickness of the end plates be substantial in order to transmit the compressive force evenly across the entire area of the plate. Also, the peripheral location of the tie rods can induce deflection of the end plates over time if they are not of sufficient thickness. Inadequate compressive forces can compromise the seals associated with the manifolds and flow fields in the central regions of the interior plates, and also compromise the electrical contact required across the surfaces of the plates and MEAs to provide the serial electrical connection among the fuel cells which make up the stack. End plates of substantial thickness however, contribute significantly to the overall weight and volume of the fuel cell stack, which is particularly undesirable in motive fuel cell applications. Also, when external tie rods are employed, each of the end plates must be greater in area than the stacked fuel cell assemblies. The amount by which the end plates protrude beyond the fuel cell assemblies depends on the thickness of the tie rods, and more importantly on the diameter of the washers, nuts and any springs threaded on the ends of tie rods, since preferably these components should not overhang the edges of end plate. Thus the use of external tie rods can increase stack volume significantly.
A compact fuel cell stack design incorporating internal tie rods which extend between the end plates through openings in the fuel cell plates and membrane electrode assemblies has been disclosed in U.S. Pat. No. 5,484,666. However, such designs increase the number of required seals in the MEA, increasing complexity, manufacturing costs and potential failure mechanisms.
Various designs in which one or more rigid compression bars extend across each end plate, the bars being connected (typically via external tie rods and fasteners) to corresponding bars at the opposite end plate, have been employed in an effort to reduce the end plate thickness and weight, and to distribute compressive forces more evenly. Such a design is described and illustrated in U.S. Pat. No. 5,486,430.
The fuel cell stack compression mechanisms described above typically utilize springs, hydraulic or pneumatic pistons, pressure pads or other resilient compressive means which cooperate with the tie rods, which are generally substantially rigid, and end plates to urge the two end plates towards each other to compress the fuel cell stack. These compression mechanisms undesirably add weight and/or volume and complexity to the fuel cell stack.
Tie rods typically add significantly to the weight of the stack and are difficult to accommodate without increasing the stack volume. The associated fasteners add to the number of different parts required to assemble a fuel cell stack.
U.S. Pat. No. 5,993,987 discloses a fuel cell stack including end plate assemblies with compression bands extending tightly around the end plate assemblies which retain and secure the fuel cell stack in its assembled state. The end plate assembly further comprises a pair of layered plates with stacks of disc springs interposed between them. The end plate assemblies preferably have rounded edges to reduce the stress on the band.
Disc springs are conventionally contacted over their inner diameter under load. For example, National Disc Springs catalogue and manual of the Rolex Company National Disc Spring Division of 385 Hillside Avenue, Hillside N.J. teaches that such disc springs should contact the load imposing surface with the disc spring's outer diameter. However, loading the disc spring by the outer diameter requires a greater amount of material to load the outer diameter resulting in greater weight and volume, increased cost of material, and reduced power density and efficiency, especially in automotive applications.
It is desirable to have a fuel cell stack with reduced weight and volume resulting in increased power density, efficiency and reduced cost. The present disclosure addresses these and associated benefits.